The Two Faces of Saccharomyces cerevisiae RAD9 Function in Homologous Recombination: Suppressor and Promoter of Genome Instability

Recombinogenic DNA damage can initiate chromosomal rearrangements that can alter gene expression or accelerate cancer progression in higher eukaryotes. Thus, there is a critical need to identify genes that suppress chromosomal rearrangements and environmental exposures that promote genetic instability. Cell cycle checkpoints modulate the cell cycle so that DNA repair occurs before the replication or segregation of damaged chromosomes. Saccharomyces cerevisiae (budding yeast) RAD9 was the first cell cycle checkpoint gene identified, which initiated intensive research studies into the mechanisms of checkpoint activation and the phenotypes of checkpoint mutants. The budding yeast Rad9 protein serves as both an adaptor and scaffold that facilitates downstream effector activation to orchestrate a DNA damage response at multiple stages of the cell cycle, which facilitate double-strand break (DSB) repair by sister chromatid recombination. However, the role of RAD9 in homologous recombination and in suppressing gross chromosomal rearrangements (GCRs) is not completely understood. In this review we discuss how RAD9 can promote genome instability resulting from aberrant DNA replication intermediates, while suppressing DSB-associated rearrangements. We also discuss possible mechanisms accounting for the synergistic increase in genomic instability in double mutants defective in both RAD9 and recombinational repair. We emphasize that while there is an overlap between checkpoint and recombinational repair pathways, RAD9 and checkpoint pathways can function independently to suppress chromosomal instability. These studies thus elucidate checkpoint mechanisms that control homologous recombination between repeated sequences.